A variety of materials are known as coating compositions for preventing corrosion of metals (primarily steels), and one of the most effective materials is a composition comprising chromic (acid) anhydride, a particulate metal (mainly zinc or aluminum), a viscosity modifier, an oxohydroxy low molecular weight ether (polyglycol) and a solvent, such as disclosed in U.S. Pat. No. 3,940,280.
This composition is ordinarily offered to users as a combination of a first component containing chromic (acid) anhydride and a pH modifier, and a second component containing a particulate metal and an oxohydroxy low molecular weight ether (such as propylene glycol). The two components are mixed prior to use, applied onto the metal surface in a specified amount and then heated at least for 0.2 second at a temperature of about 200.degree. C. or more to form the coating film.
This composition exhibits superior anticorrosive effect against salt water on steel, but not the same effect against fresh water. Because it contains hexavalent chromium, some consideration may need to be given to environmental pollution. For example, the hexavalent chromium contained in the waste wash water discharged in the course of composition use might first be reduced.
The mechanism of the anticorrosion activity of this composition is, although it is not entirely understood, believed that the chromium oxide (reduction product of the chromic (acid) anhydride, mainly Cr.sub.2 O.sub.3) acts as bonding material (matrix) for the metal powder, and thus a coating layer is formed on the surface of the metal body, the said matrix having a certain degree of electric conductivity, and the hexavalent chromium remaining in the chromium oxide keeps the metal powder active, thus making the metal powder exhibit the sacrificial protection effect. However, since the conductivity of the chromium oxide itself is very low, the sacrificial protection effect is satisfactorily exhibited in the salt water environments where electrolytes are present in large amounts. But, this same effect is not produced in fresh water environments because the conductivity of the chromium oxide, per se, is low, and the remaining hexavalent chromium is easily leached out.
An anticorrosive coating composition has been sought that will be effective in fresh water as well as that will be highly acceptable environmentally, by seeking a bonding material that would replace the chromic acid. Numerous compositions have been compounded of various molybdic acid compounds, phosphoric acid compounds and boric acid compounds; exposure tests have been conducted; and it has been found that boric acid compounds were effective, by substituting a boric acid compound for a part of the chromic (acid) anhydride.